Portland General Electric announced plans late today to build two new natural gas power plants by 2015 and spend more than $500 million to keep its controversial Boardman coal-fired power plant burning for three more decades.

Oregon's largest electric utility also unveiled plans to ramp up conservation and wind power to help meet growing demand.

PGE's latest two-decade energy strategy would fall well short of Oregon's 2020 goals for cutting carbon emissions to combat global warming.

Environmental groups and other critics quickly denounced the plan, calling it risky for ratepayers and the environment, while business groups said it makes sense for PGE to bank on reliable power.

PGE officials said the 314-page draft plan, now out for public comment through Oct. 5, provides the best combination of cost and reliability of the 15 power supply scenarios it considered for its 817,000 business and residential ratepayers.

Boardman is PGE's cheapest source of day-to-day power after hydroelectric power and wind, PGE spokesman Steve Corson said.

The plan "meets our customers' priorities for energy efficiency and renewables, and at the same time makes good use of our existing resources," Corson said.

But critics said investing up to $560 million in pollution control upgrades at the Boardman coal plant would put ratepayers at risk if federal curbs on carbon emissions force the plant to close anyway.

They also questioned the plan's assumption that meeting Oregon's carbon reduction goals would require adding nuclear power instead of further increasing conservation and renewable power.

"That on its face is a ridiculous scenario," said Bob Jenks, executive director of the Citizens' Utility Board of Oregon, a ratepayer advocacy group.

PGE's projection of conservation savings remained roughly the same across all the many scenarios it considered, Jenks and other critics noted.

The utility said its conservation goals -- targeted to meet half of the growth in demand through 2020 -- are the most it can reliably achieve given current technologies.

Oregon utilities issue their energy strategies, known as "integrated resource plans," every two years. PGE's new plan will undergo particularly heavy scrutiny this time because of recent state requirements for more pollution controls at Boardman and the debate over global warming.

PGE serves more than 40 percent of Oregon's population. It projects growth in demand of 2.3 percent a year through 2020.

The utility also faces a drop in electricity supply from some Columbia River hydroelectric dams. And it wants to reduce its reliance on buying electricity in the open market, where prices can see-saw unpredictably.

The company is considering building two gas-fired power plants, including a large plant at Boardman and a smaller unit on the lower Columbia River. It's not releasing cost estimates or rate impacts. But the tab for PGE's last large gas plant was $285 million by the time it fired up in 2007.

PGE also will explore adding enough wind power to supply about 100,000 residential customers through new wind farms in Sherman, Jefferson or Umatilla counties.

That would nearly equal the planned capacity at the utility's Biglow Canyon wind farm, expected to cost $1 billion total.

And it would help meet Oregon's renewable power standard, which requires 15 percent of energy to come from renewables by 2015.

Keeping the Boardman plant open would require a half-billion investment in controls to reduce emissions of nitrogen oxides and sulfur dioxide under Oregon's new requirements.

The plant makes up about 25 percent of the utility's generation.

The Sierra Club and other environmental groups have sued PGE over the adequacy of those controls. Shuttering Boardman is also a key part of the Sierra Club's national "Beyond Coal" campaign.

Robin Everett, the campaign's Northwest representative, said PGE "doesn't want to do the hard stuff" it would take to replace the coal plant's electrical supply, including pursuing more renewables and low-cost energy efficiency.

A similar argument played out earlier this year in the Legislature, which rejected efforts to put teeth into Oregon's greenhouse gas reduction goals. PGE and other utilities and business groups said meeting the goals -- 10 percent below 1990 levels by 2020 -- was too ambitious and costly.

John Ledger, a vice president with Associated Oregon Industries, said Friday that PGE has to bank on reliable power generation. And when it comes to renewables, carbon costs and future technology, "Everything's up in the air right now."

PGE analysts don't think Boardman's supply could be met with wind, which can't be counted on at all times, Corson said. During Oregon's recent heat wave when power demands peaked, wind generation stalled in the stagnant air.

Shutting down Boardman would require building even more gas plants, Corson said, exposing customers to high rates if gas prices spike.

Taking the further step of meeting Oregon's carbon reduction standards would require a reliable, carbon-free source. PGE analysts think the candidate for that is nuclear power. That's now banned under Oregon law, and would have to come from out of state.

Oregon's Public Utility Commission will ultimately have to decide on the plan.